The present invention relates to an apparatus for tensioning and securing a lashing strap, and more particularly to a tensioning apparatus of the type that includes a basic frame to which one end of the lashing strip is attached, a wind-up spindle that is rotatably mounted on the frame and that receives the other end of the lashing strap, and a ratchet mechanism to secure the spindle from reverse rotation with respect to the frame, the ratchet mechanism including a ratchet wheel and at least two locking pawls which can be displaced under pressure toward the ratchet wheel and which can be brought into engagement with the ratchet wheel. In such a tensioning apparatus, which may also be called a tensioning ratchet, the lashing tension is created in the lashing strap as a result of the strap being shortened due to being wound up.
To prevent rotation in the reverse direction during the tensioning process, the wind-up spindle can be blocked in the direction opposite to its tensioning direction by means of the ratchet mechanism. This ratchet mechanism also locks the wind-up spindle in its end position when the lashing strap is completely tensioned or lashed.
Such ratchet mechanisms are disclosed, for example, in Richter and Von Vos, Bauelemente der Feinmechanik [Precision Mechanism Components], 9th Edition, 1964, page 262. According to this publication, a ratchet mechanism is composed of a locking member and a lockable member. It is also known to reduce the ratchet movement steps by providing several locking means or detent latches (see Lueger, Lexikon der Technik [Encyclopedia of Technology], Volume 1, 1960, page 530).
The ratchet mechanisms mentioned in Richter and Von Voss (supra), in particular, make it possible to introduce the tensioning force into the strap in steps by rotating the wind-up spindle in the tensioning direction, while simultaneously ensuring that the wind-up spindle will be unable to turn in the reverse direction. In ratchet mechanisms of this type, it is customary to configure the locking members not as detent latches but as locking pawls. The detent latches are mounted so as to be pivotal and are biased by a spring element. In order to transfer great forces, the detent latches and the spring elements must be made very large. In contrast thereto, when locking pawls are used as the locking elements they permit, in principle, a space saving configuration. Moreover, high forces can be transmitted by way of such locking pawls.
When in the engaged position, the locking pawls engage in teeth carried on the ratchet wheel. These teeth are oriented in such a way that, in order to lock, the locking pawls grip behind the steep leading edges of the ratchet teeth, while rotation of the wheel causes the locking pawls to slide over the rounded rear tooth edges.
With every locking step, the locking pawls move in order to pass over a ratchet tooth. For this reason, the locking pawls are mounted on the basic frame of the apparatus, preferably at the side walls of the frame, so as to perform a linear or translatory movement. Thus the frame side walls must be designed in such a way that the path required for the translatory movement is available. At the end of the locking step and after passing over the ratchet tooth, the locking pawls undergo another translatory movement radially back into the teeth.
To be able to lock at smaller steps that differ from the tooth pitch of the ratchet wheel, or in order to be able to absorb lashing forces that a locking pawl or detent latch on its own would be unable to absorb, the prior art provides for the arrangement of a plurality of locking pawls that are distributed radially around the circumference of the ratchet wheel. The structural height of the basic frame must be increased with the number of locking pawls distributed radially around the circumference of the ratchet wheel, since the structural height of the side wall of the frame must be great enough to accommodate the translatory movement of each locking pawl.
To reduce the structural height of such a tensioning apparatus employing two locking pawls, it is therefore desirable for the locking pawls to be oriented in the manner of the legs of an acute angle, or better still to be oriented as parallel as possible to one another. A tensioning apparatus having such a parallel arrangement of locking pawls is disclosed in German Patent 3,344,487, corresponding to U.S. Pat. No. 4,570,305. The drawback of this prior art apparatus is the fact that the locking element is divided into two parts in order to obtain the two parallel locking pawls. Two such parallel locking pawls are able to step only in small tooth pitches. If the tooth pitch is too large, the pawls would have to be designed unusually large and thus high.
Another drawback in the prior art apparatus is that the locking pawls are always in engagement with the same succession of ratchet teeth. A particularly good locking effect is attained with radially arranged locking pawls whenever the steep leading edge of the ratchet tooth behind which a locking pawl engages lies completely, and over its full area, against the side of the locking pawl, thus generating pressure at the area of engagement. If the locking pawl does not extend precisely radially to the ratchet wheel, the steep leading edge of the tooth lies against the end of the locking pawl, which causes the locking pawl to press against the steep tooth edge only at certain points. This point-like engagement may lead to the gradual flattening of the steep tooth edge and, in the worst case, to breakage of the tooth edge.
Moreover, with two parallel locking pawls, only a small tooth angle ca be achieved between the steep leading edge of a tooth and the rounded trailing edge if the above-described risk of a point-shaped load at the end of the locking pawl is to be avoided (see Richter and Von Voss, supra).
If a plurality of locking pawls is to be employed so as to absorb extremely high lashing forces, it is advantageous, in principle, to allow them to engage ratchet teeth at several locations distributed around the circumference of the ratchet wheel. In this way, the lashing force is distributed better to the individual locking pawls. However, if the locking pawls are mounted in a common guide, they are able to engage the teeth only at one location